Method of Providing Access to an IP Multimedia Subsystem

ABSTRACT

A method of providing access to an IP multimedia subsystem (IMS) for a subscriber (UE) comprises the steps of providing an home interface entity (HGGSN) with access network data (MCC′,MNC′) identifying the access communication network (AN), sending a message comprising the access network data (MCC′,MNC′) from an home interface entity (HGGSN) to serving means (HSS), comparing the home network data (MCC,MNC) and the access network data (MCC′,MNC′), checking if the subscriber (UE) is allowed to roam based on a set of predefined criteria when the result of the comparing step is false, and sending a message comprising the access network data (MCC′,MNC′) to at least one of call state control functionality means (CSCF) when the result of the checking step is true.

The present invention relates to a method of providing access to an IPmultimedia subsystem for a subscriber communicating in an accesscommunication network according to the preamble of claim 1.

Mobile communication network GSM were designed on the basis of switchedcircuit (CS) connection. The advent of GPRS (General Packet RadioService) introduced packet switched (PS) connection from mobileterminals and network servers. However, the services offered to the userwere still determined by the network operator.

The third generation partnership project (3GPP) has defined a newstandardized architecture known as Internet Protocol (IP) MultimediaSubsystem or IMS for offering multimedia services in a packet switchedbased domain to end users whereby the network operator accessing to IMSservices controls the services offered by the IMS.

IMS is access agnostic to the extent of the network requesting access toits services so that in a multi access environment it ensures servicesavailability to all access networks, subject to the limitations of theaccess network or the specific user requesting access to IMS.

The core network of an IMS architecture comprises database elements andIMS control elements.

Home subscriber server (HSS) is the main database element of IMS and isresponsible for holding user-related data such as user identification,numbering, addressing, security, location management and user profileinformation.

IMS control elements are call session control function (CSCF) serverswhich comprise proxy-CSCF (P-CSCF) which is the first point of contactfor users within the IMS and is responsible for forwarding messages,particularly session initial protocol (SIP) messages, from the user tothe other CSCF elements, serving-CSCF (S-CSCF) which performsregistration, session control for registered end points, interactionwith service platforms/application servers of IMS and interfacing to HSSfor users registered therein, and interrogating-CSCF (I-CSCF) which isthe contact point for external networks and is responsible for findingthe appropriate S-CSCF from HSS.

In IMS roaming the packet network resources of the visited mobiletelecommunication network in which a user in registered are used toconnect the user to IMS core network, which may reside in the homemobile network (HPLMN) or in the visited mobile network (VPLMN). In anycase, all the core elements of the IMS are always located at the homenetwork and the subscriber is always registered with the S-CSCF of thehome network for the purposes of charging and interfacing to the HSS.

When an IMS subscriber is located at his home GPRS/IMS network, heaccesses IMS services through a home network access point, such as thegateway GPRS support node (GGSN).

A roaming scenario supported by IMS is the so called Home GGSN (HGGSN)model where the roaming subscriber uses the home network access pointHGGSN to access IMS services. In this scenario, the subscriber islocated under a visited network while user plane and control plane dataare sent via the HGGSN located in the subscriber's home network.

This is a typical GPRS roaming situation where the access point HGGSNfor accessing IMS services is in the home network and a GPRS tunnellingprotocol (GTP) is used across the inter-operator backbone network of thehome and visited network.

One of the major business requirements for an operator is to apply aproper tariff to the user that are in roaming condition, both outgoingroamers as well as the incoming roamers. In particular the requirementis to identify whether a user is in roaming condition or not andeventually in which network the user is actually roaming. Thisrequirement is not specifically addressed by 3GPP or any otherstandards.

A workaround would be possible by correlating HGGSN generated charginginformation and IMS generated charging information. However, this wouldeventually work for offline charging but it does not work for onlinecharging since no OCSs (Online Charging Systems) have the ability toperform the correlation process in real-time.

It is therefore an object of the present invention to provide a methodand a system for accessing to IMS so as to solve the above mentionedproblem.

According to the present invention, this object is achieved by a methodaccording to claim 1.

The method according to the invention make the IMS core network aware ofthe roaming condition of the subscriber and eventually in which networkthe subscriber is located. Moreover a mechanism to propagate the roaminginformation within IMS core network and to the application serversresiding on the service layer of IMS ensures roaming awareness to allrelevant IMS network entities.

With IMS becoming roaming aware it is possible to authorize a user toaccess IMS and IMS services depending on the roaming condition andeventually depending on the visited network where the subscriber iscurrently located. Finally, differentiated charging based on roaminginformation becomes possible for all services deployed on top of IMS.

Furthermore, the proposed solution works regardless the chargingarchitecture and methods, both online and offline charging.

In the following, preferred embodiments of the invention will bedescribed in detail with reference to the accompanying drawings, inwhich:

FIG. 1 shows schematically the elements of a communication system whichallows communication between a subscriber terminal and an IP multimediasubsystem IMS in accordance with the method of the invention, and

FIG. 2 shows the data exchanged between elements of the communicationsystem of FIG. 1 to provide access to the IP multimedia subsystem IMSfor the subscriber terminal.

Referring to the attached figures, a subscriber UE communicates, bymeans of a mobile terminal, MT, in an access communication network AN,for example a mobile GPRS or UMTS telecommunications network andrequests access to an IP multimedia subsystem IMS through an interfaceentity HGGSN located in the home communication network HN of thesubscriber UE.

In the example, the interface entity HGGSN is gateway GPRS support node.

A serving support entity SGSN, such as a serving GPRS support node, isassociated with the access network AN and allows packet data to betransmitted between the subscriber UE and the access network AN.

The IP multimedia subsystem IMS is located in the home network HN of thesubscriber UE and comprises serving means HSS for home subscribersstoring profile data of the subscriber UE and home network data of thehome communication network HN and call state control functionality meansCSCF for controlling service calls within the IP multimedia subsystemIMS.

According to a preferred embodiment, the network data comprise a mobilecountry code and a mobile network code. In the example, the home networkdata comprise a mobile country code MCC and a mobile network code MNC.

The IMS comprises also at least one application server AS which offerservices to subscribers accessing to the IP multimedia subsystem IMS.

The home subscriber serving means HSS store subscriber-related data suchas user identification, numbering, addressing, security, locationmanagement and user profile information. The home serving means HSSincludes HLR elements, such as subscriber data and authentication data,and a new functionality as location register and IMS subscription andauthentication data which are referred to as the user mobility server(UMS).

A subscription locator function SLF database may be provided in the IMSnetwork for obtaining the HSS which stores the subscriber data of thesubscriber UE, when more than one HSS is present in the IMS network.

According to a preferred embodiment, the call state controlfunctionality means CSCF comprise an access entity P-CSCF, a controlentity I-CSCF and a serving entity S-CSCF.

The access entity P-CSCF comprises is responsible for forwarding SIPmessages from the user UE to the other call state control functionalitymeans CSCF. The access entity P-CSCF is the entry point towards the IPmultimedia subsystem IMS from the access network AN and its assignmentto a user UE is determined by the configuration of the mobile networkrequesting access to the IP multimedia subsystem IMS. As known in theart, in the case of UMTS/GPRS the allocation of a P-CSCF takes place atthe packet data protocol PDP context activation.

The control entity I-CSCF is adapted to find the appropriate servingentity S-CSCF on the basis of the information stored in HSS. In fact,the primary function of the I-CSCF is the query of the HSS to know theS-CSCF which a user is to be registered with.

The serving entity S-CSCF performs registration, session control forregistered end points, interaction with service platforms/applicationservers of IMS and interfacing to HSS.

In order to establish a communication session, the mobile terminal ofthe subscriber UE generates a PDP context which defines the parametersthat support the flow of data traffic to and from the mobile terminal.

Particularly, a register message for registering to the IP multimediasubsystem IMS is sent from the terminal of the subscriber UE to at leastone of the call state control functionality means CSCF.

When the subscriber UE wishes to access to the IP multimedia subsystemIMS, the home interface entity HGGSN is provided with the access networkdata identifying the access network AN.

Afterwards, the home interface means HGGSN send to the home subscriberserving means HSS a message, such as a RADIUS message, comprising suchaccess network data.

Subsequently, the home network data and the access network data arecompared to ascertain whether the subscriber is roaming or not.

When the result of the comparison is false, that is the home networkdata and the access network data mismatch, the subscriber UE is roamingin the access network AN and therefore the method provides that it ischecked if the subscriber UE is allowed to roam based on a set ofpredefined criteria.

On the contrary, when the result of the comparison is true, that is thehome network data match the access network data, the subscriber UE isnot roaming so that the access network AN where it is communicating isits home network HN.

When the result of the check is true so that the subscriber is allowedto roam a message comprising the access network data is sent to at leastone of the call state control functionality means CSCF.

On the contrary, when the result of the check is false so the subscriberis not allowed to roam, a error message is sent to the subscriberterminal.

Advantageously, the home network data and the access network datacomprise at least a respective mobile country code MCC,MCC′. In thiscase, the result of comparison is false when the mobile country codesMCC,MCC′ differs.

More advantageously, the home network data and the access network datacomprise a respective mobile network code MNC,MNC′. In this case, theresult of the comparison is false even when the mobile country codesMCC,MCC′, match and the mobile network codes MNC,MNC′ differ.

The mobile country codes may be sufficient for establishing whether thesubscriber UE is roaming or not, for example if they differ. However, inorder to know the specific access network AN where the subscriber UE isroaming, both the country code MCC and the network code MNC are requiredsince the same network code MNC may be used in different countries and,on the other hand, several different network may exist in each country.

Preferably, the checking is performed by comparing the access networkdata MCC′,MNC′ with a predefined list of allowed access network data,such as a list of access network data corresponding to operator beingpartners of the home network operator of the subscriber.

The access network data MCC′,MNC′ may be stored into the home subscriberserving means HSS.

According to a preferred embodiment, the home interface entity HGGSNreceives the access network data, such as mobile country code MCC′ andmobile network code MNC′, from the serving support entity SGSN in theaccess network AN.

Alternatively, the home interface entity HGGSN comprises storing means(not shown in the figures) for storing a plurality of IP addressesIP_(n) and a corresponding plurality of network data MCC_(n),MNC_(n). Inthis case, the home interface entity HGGSN receives the IP_AN address ofthe serving support entity SGSN and associates to such IP address IP_ANthe access network data MCC′ and MNC′.

For example, a mapping table may be provided in the home interfaceentity HGGSN for associating to the IP address IP_AN of the servingsupport entity SGSN the corresponding pair of codes MCC′ and MNC′. Itmay be assumed that for all roaming partners of a given operator whichare known to not send the MCC′ and MNC′, codes, there is a row in themapping table which consists of an IP subnet and the corresponding MCC′and MNC′ codes.

The access network data MCC,MNC may be transported within an appropriateRADIUS attribute, such as for example the 3GPP-SGSN-MCC-MNC attributevalue pair (AVP).

In case of early IMS security, the message sent from the home interfaceentity HGGSN to the serving means HSS is the same RADIUS message used tosend the IMSI (International Mobile Subscriber Identifier)/IP addresssecurity binding.

According to a preferred embodiment, the serving support entity SGSN ofthe mobile network AN where the subscriber UE is communicating withsends the MCC′ and MNC′ codes to the home interface entity HGGSN, forexample by a GTP message.

The RADIUS message containing access network data MCC′, MNC′ reaches thehome subscriber means HSS, specifically the UMS register, where the twocode MCC′,MNC′ are stored. The access network data or roaminginformation MNC′ and MCC′ are preferably stored in the HSS,particularly, in the UMS, in association with the user identity data ofthe subscriber UE, such as the IMSI or MSISDN.

Subsequently, as stated above, depending on the comparison of the homenetwork data MCC,MNC and the access network data MCC′,MNC′ and on a setof predefined criteria, the home subscriber serving means HSS authorizeor deny access of the subscriber UE to the IP multimedia subsystem IMSand send this information to the serving entity S-CSCF via an interface,such as a Cx interface or by other equivalent means.

In particular, if the roaming information MNC′ and MCC′ stored in theserving means HSS for the subscriber UE communicating in the mobilenetwork AN differ from the MNC and MCC codes of the home network of theIP multimedia subsystem IMS, the serving means HSS recognize that thesubscriber UE is roaming in a visited network, that is the accessnetwork AN. In this case, the serving means HSS checks if the subscriberUE is allowed to roam and, if it is allowed to roam, checks if thesubscriber UE is allowed to roam in the access network AN.

Additional provisions may be set up for allowing or denyingauthorization to the IP multimedia subsystem IMS to a specificsubscriber UE depending on subscriber identity data and the particularmobile network AN, identified by the roaming information MCC′,MNC′ wherethe subscriber UE 1 is roaming.

It should be noted that there may be an Authentication AuthorizationAccounting (AAA) server or a Diameter translation agent or an AAA serveracting as Diameter translation agent between the home interface entityHGGSN and the serving means HSS.

The serving entity S-CSCF propagates the roaming information MCC and MNCwithin the IMS core entities and the possible trusted applicationservers, for example the application server AS, using an appropriateheader, such as a P-Visited-Network-Id header.

By this way, all IMS core entities include MCC and MNC code in generatedcharging information, for both online and offline charging.

FIG. 2 illustrates an application of the method of the present inventionfor the home GGSN model where early IMS security is used.

Firstly, the HSS checks if the subscriber UE is allowed to roam uponreception of a User-Authorization-Request (UAR) message. If thesubscriber UE is not allowed to roam or if it is not allowed to roam inthe mobile network AN, the HSS rejects the authorization request bysending an error message, that is User-Authorization-Answer (UAA)including an error code such as “DIAMETER_ERROR_ROAMING_NOT_ALLOWED”.

This check is performed based on the roaming information, MCC′ and MCC′codes, stored in HSS for the subscriber UE subscription, i.e. the MCC′and MNC′ received from the home interface entity HGGSN.

It comes out that the IMS access and registration of the subscriber UEmay be accepted or rejected simply based on roaming status(allowed/denied or not_allowed) or based on a list of networks whereroaming is allowed.

Where the home GGSN roaming model is active in HSS and the roaminginformation MCC′ and MNC′ is not available at the initial IMSregistration of a subscriber, for example if the roaming information isnot received from the home interface entity HGGSN, the operator may beable to decide whether to accept or reject the IMS registration of thesubscriber UE based on configuration parameters.

In case the roaming information MCC′ and MNC′ is available as stored inHSS when the SAR message is received from the HSS, the HSS sends thesaid roaming information MCC′ and MNC′ in SAA message to the S-CSCF.

The roaming information MCC′ and MNC′ may also be sent within a3GPP-SGSN-MCC-MNC attribute derived from the 3GPP specific RADIUSattribute defined in 3GPP TS 29.061 or within any other appropriateattribute.

Where the roaming information MCC′ and MNC′ is received from the HSS,the S-CSCF populates an appropriate header such as, for example, theP-Visited-Network-Id Header in the 200 OK of a message, for example theREGISTER message.

IMS entities I-CSCF and P-CSCF in the path to the subscribe UE storethis roaming information MCC′ and MNC′ so that all entities of the IPmultimedia subsystem IMS are aware of the roaming condition of thesubscriber UE.

The access entity P-CSCF strips the P-Visited-Network header or anyother P-Header used to transport the roaming information MCC′ and MNC′before sending any message to the subscriber UE. TheP-Visited-Network-Id header may be populated as a dotted separatedconcatenation of the MNC and MCC as follow:mnc<MNC>.mcc<MCC>.3gppnetwork.org.

As it will be described more in detail in the following, the accessentity P-CSCF receives the roaming information MNC′ and MCC′ also withinthe notification to the register event.

The serving entity S-CSCF includes the P-Visited-Network-Id header orany other P-Header appropriate to carry the roaming information MCC′ andMNC′ in third party REGISTER message sent to trusted AS.

Particularly, if an application server AS is interested in receivingchanges in roaming status, it may subscribe to a registration eventpackage. In this case, the roaming information MNC′ and MCC′ is carriedwithin a P-Visited-Network-Id header or any other appropriate P-Headerincluded in the NOTIFY request. The AS stores the received roaminginformation MNC′ and MCC′ associated to the registered public useridentities indicated within the body of the NOTIFY.

The serving entity S-CSCF or an IP multimedia gateway (IP-GW) includesthe roaming information MCC′ and MNC′ when it generates online chargingrequests.

When they generates offline charging requests or Charging Data Record(CDR), call session control function means CSCF includes the roaminginformation MNC′ and MCC′.

In case the HSS receives a new roaming information MNC′ and MCC′, forexample because the subscriber UE changed its roaming status, the HSSupdates the S-CSCF by sending a Push-Profile-Request (PPR) including thenew roaming information MNC′ and MCC′ for instance within a3GPP-SGSN-MCC-MNC attribute.

The S-CSCF considers the receipt of this attribute as a change inregistration status of the subscriber UE and stars notification to theapplication servers AS and the other authorized entities of the IMS,such as the P-CSCF, that subscribed to the registration event package.

It should be noted that S-CSCF may record the changes in roaming statusinto the offline charging requests or CDRs it generates or may triggeran Accounting-Request (ACR) on an appropriate interface, such as the Rfinterface, including the new roaming information MNC′ and MCC′.

If online charging is performed for the subscriber, the S-CSCF mayreport the new roaming information MNC′ and MCC′ together withused-units, such as the quota consumed until the change in roamingstatus, to the OCS(?).

Finally, the subscribers to the registration event, such as the P-CSCFand the AS, receive the notification of changing in roaming status andreplace the previously stored roaming information MNC′ and MCC′ with thenew roaming information MNC′ and MCC′ included in the latest receivedP-Visited-Network-Id header or any other appropriate P-Header that maytransports the roaming information. As a result of the above definedprocedure the SAA and PPR commands defined in 3GPP TS 29.229 are addedwith the roaming information MNC and MCC.

However, any other appropriate attribute may be used to carry theroaming information MNC′ and MCC′.

From the discussion above, it should be clear to those skilled in theart that the embodiment of the method and system described allows theIMS entities to become aware of the roaming condition and in whichnetwork a subscriber requesting access to the IMS network is located.

Moreover, the solution described allow the roaming information to bepropagated within IMS entities and to the application servers residingon the service layer of IMS thereby ensuring roaming awareness to allrelevant IMS network entities.

With IMS becoming roaming aware it is possible to authorize a user toaccess IMS and IMS services depending on the roaming condition andeventually depending on the visited network where the subscriber iscurrently located.

Finally, differentiated charging based on roaming information becomespossible for all services deployed on top of IMS. Furthermore, asdiscussed above, the invention works both with online and offlinecharging methods.

The present invention allows standards IMS network to be used withoutneed of a specific proprietary node or function within the IMS networkexcept for the implementation of the specific method of the invention asdescribed for the roaming awareness of the IMS network and its entities.The solution according to the present invention may therefore beimplemented and integrated in existing IMS networks without requiringany support from the roaming partner network.

While the invention has been described with reference to a preferredembodiment, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the scope of the invention as defined by the appended claims.

1. Method of providing access to an IP multimedia subsystem (IMS) for asubscriber (UE) communicating in an access communication network (AN)and requesting access to said IP multimedia subsystem (IMS) through ahome interface entity (HGGSN) located in the home communication network(HN) of said subscriber (UE), said IP multimedia subsystem (IMS) beinglocated in said home communication network (HN) and comprising servingmeans (HSS) for home subscribers storing profile data of said subscriber(UE) and home network data (MCC,MNC) of said home communication network(HN) and call state control functionality means (CSCF) controllingservice calls within said IP multimedia subsystem (IMS), the methodcomprising the steps of: providing said home interface entity (HGGSN)with access network data (MCC′,MNC′) identifying said accesscommunication network (AN) sending a message comprising said accessnetwork data (MCC′,MNC′) from said home interface entity (HGGSN) to saidserving means (HSS), comparing said home network data (MCC′,MNC′) andsaid access network data (MCC′,MNC′), checking if said subscriber (UE)is allowed to roam based on a set of predefined criteria when the resultof said comparing step is false, sending a message comprising saidaccess network data (MCC′,MNC′) to at least one of said call statecontrol functionality means (CSCF) when the result of said checking stepis true.
 2. Method of providing access to an IP multimedia subsystem(IMS) according to claim 1, wherein the result of said comparing step isfalse when said home network data (MCC,MNC) and said access network data(MCC′,MNC′) mismatch.
 3. Method of providing access to an IP multimediasubsystem (IMS) according to claim 1, wherein said home network data(MCC,MNC) and said access network data (MCC′,MNC′) comprise at least arespective mobile country code (MCC,MCC′) , the result of said comparingstep being false when said mobile country codes (MCC,MCC′) differ. 4.Method of providing access to an IP multimedia subsystem (IMS) accordingto claim 2, wherein said home network data (MCC,MNC) and said accessnetwork data (MCC′,MNC′) comprise at least a respective mobile countrycode (MCC,MCC′), the result of said comparing step being false when saidmobile country codes (MCC,MCC′) differ.
 5. Method of providing access toan IP multimedia subsystem (IMS) according to claim 1, wherein said homenetwork data (MCC,MNC) and said access network data (MCC′,MNC′) comprisea respective mobile network code (MNC,MNC′), the result of saidcomparing step being false when said mobile country codes (MCC,MCC′)match and said mobile network codes (MNC,MNC′) differ.
 6. Method ofproviding access to an IP multimedia subsystem (IMS) according to claim2, wherein said home network data (MCC,MNC) and said access network data(MCC′,MNC′) comprise a respective mobile network code (MNC,MNC′), theresult of said comparing step being false when said mobile country codes(MCC,MCC′) match and said mobile network codes (MNC,MNC′) differ. 7.Method of providing access to an IP multimedia subsystem (IMS) accordingto claim 1, wherein said checking step comprises the step of comparingsaid access network data (MCC′,MNC′) with a predefined list of allowedaccess network data.
 8. Method of providing access to an IP multimediasubsystem (IMS) according to claim 1, further comprising the step ofsending an error message to the subscriber terminal when the result ofsaid checking step is false.
 9. Method of providing access to an IPmultimedia subsystem (IMS) according to claim 1, comprising, before theproviding step, the step of sending a register message for registeringto said IP multimedia subsystem (INS) from the subscriber terminal to atleast one of said call state control functionality means (CSCF). 10.Method of providing access to an IP multimedia subsystem (IMS) accordingto claim 1, comprising, before the comparing step, the step of storingsaid access network data (MCC′,MNC′) into said serving means (HSS). 11.Method of providing access to an IP multimedia subsystem (IMS) accordingto claim 1, wherein said subscriber (UE) communicates with said accesscommunication network (AN) through a serving support entity (SGSN). 12.Method of providing access to an IP multimedia subsystem (IMS) accordingto claim 11, wherein said providing step comprises the step of receivingat said interface entity (HGGSN) said access network data (MCC′,MNC′)from said serving support entity (SGSN).
 13. Method of providing accessto an IP multimedia subsystem (IMS) according to claim 11, wherein saidinterface entity (HGGSN) comprises storing means for storing a pluralityof IP addresses (IP_(n)) and a corresponding plurality of network data(MCC_(n),MNC_(n)), said providing step comprises the steps of: receivingat said interface entity (HGGSN) the IP address (IP) of said servingsupport entity (SGSN), associating to said IP address (IP) of saidserving support entity (SGSN) said access network data (MCC′,MNC′).